Currently, liquid crystal has been applied in information display and some progress has been made in optical communication (S. T. Wu, D. K. Yang. Reflective Liquid Crystal Displays. Wiley, 2001). In recent years, applications of liquid crystal compounds have significantly broadened to various types of display devices, electro-optical devices, electronic components, sensors and the like. For these purposes, a plurality of different structures has been proposed, particularly in nematic phase liquid crystals. Nematic phase liquid crystal compound so far has been widely used in flat panel displays, especially in thin film transistor (TFT) active matrix liquid crystal display (AMLCD) systems.
It has been a long path of development since the discovery of liquid crystals. In 1888, Friedrich Reinitzer, an Austrian botanist, discovered the first liquid crystal material, i.e., cholesteryl benzoate. Manguin invented a rubbing orientation for the production of single domain liquid crystal and initiated the research on optical anisotropy in 1917. E. Bose established Swarm doctrine in 1909, which was supported by experiments of L. S. Ormstein and F. Zernike et al (1918) and later was explained as statistical fluctuations by De Gennes. In 1933, G. W. Oseen and H. Zocher founded continuum theory which was modified by F. C. Frank later in 1958. M. Born (1916) and K. Lichtennecker (1926) found and studied liquid crystal dielectric anisotropy respectively. In 1932, W. Kast accordingly divided the nematic phase into positive and negative two categories. In 1927, V. Freedericksz and V. Zolinao discovered that nematic liquid crystal would be deformed and presented a voltage threshold (Freederichsz change) in the electric field or magnetic field. The discovery provides a basis for liquid crystal displays.
In 1968, R. Williams in Radio Corporation of America (RCA) found that nematic phase liquid crystals formed domain structure and had light scattering phenomenon in the electric field. G. H. Heilmeir then developed a dynamic scattering mode, the first liquid crystal display in the world. In the early 1970s, Helfrich and Schadt invented twist-nematic (TN) principle. The combination of the TN photoelectric effect and integrated circuit made a display device (TN-LCD), which has opened up a broad prospect for the application of liquid crystals. Since then, due to the development of large scale integrated circuits and the advancement of liquid crystal material, the liquid crystal display, in the art, has made a breakthrough. Super Twist Nematic (STN) mode was proposed successively by T. Scheffer et al. in 1983 through 1985 and an AMLCD mode proposed by P. Brody in 1972 were re-adopted. Conventional TN-LCD technology has been transferred to STN-LCD and TFT-LCD technologies. Although STN scanning lines can be up to 768 lines, there are still some shortcomings such as response speed, viewing angle and gray scale when the temperature rises. Therefore, for a large panel, high information content, high color quality display, an active matrix display becomes the first choice. TFT-LCD has been widely used in direct-view TV, large-screen projection television, computer monitor and certain military instrument display. It is believed that TFT-LCD technologies will have even broader applications.
There are two types of “active matrix” structures: firstly, metal oxide semiconductor (MOS) on a silicon wafer as the substrate. Secondly, thin film transistor (TFT) fabricated on a glass substrate.
Monocrystalline silicon as the substrate has a limitation with its display size due to the fact that there were many problems at junctions of each part of the display unit or module assembly. Accordingly, the second type of TFT active matrix is promising. The photoelectric display effect is generally TFT-TN mode. TFT substrate includes a compound semiconductor, such as of CdSe, a polycrystalline silicon as well as amorphous silicon.
For small and medium-size displays such as monitors and smart phones, the in-plane switching (IPS) and fringe field switching (FFS) mode is very interesting for its large viewing angle compared with TFT-TN display. With respect to the IPS, FFS has a higher transmittance. The liquid crystal mixture of those displays lies in the compounds with positive dielectric anisotropy, and optionally, neutral compounds.
It is desirable that the LCs inside the display cell structure should have the following advantages:
1. wide temperature range of nematic phase (in particular, low-temperature range)
2. fast response time (outdoor use, automobiles, avionics) switching at very low temperatures
3. superior resistance to UV radiation (longer service life)
4. low threshold voltage (to save energy)
5. high transmittance.